The word pneumonoultramicroscopicsilicovolcanoconiosis is often cited as the longest word in the English language. It is the longest word found in a standard dictionary, but the full name for Tryptophan synthetase is longer, but not in a common dictionary. Some might wonder about the word I since it consists of one letter, too. In sound, a is shorter because it is a monophthong (consists of one vowel), while I is a diphthong.  Both do consist of one letter in the English writing system, and in most fonts I is the narrowest letter.  However, since all languages have spoken forms, but not all have written forms, the writing system is actually a poor measure of word length, and a is best considered the shortest word in English. That is, a word doesn't have to be written in order to exist in a language. It simply has to be a free form.